Jen from Among the Hidden was the first protagonist in Margaret Peterson Haddix’s Shadow Children series. She is a twelve-year-old girl living in a dystopic post-apocalyptic world where the government limits families to only two children. In Jen's world, the “third children” born in each family, who are usually female, are illegal and must remain hidden and suffer terrible consequences if they are discovered.
Jen is a brave and resilient girl who takes on the responsibility of fighting for the rights of third children like her all over the country. She is also deeply courageous and compassionate and she constantly displays both traits in the risky situations she faces in her journey to discover her true identity.
To answer the question, Jen from Among the Hidden is twelve years old.